A rotary kneading screw for use in an extruder is disclosed in Applicant's prior U.S. application Ser. No. 279,823. This prior rotary kneading screw comprises a plurality of kneading recesses formed along a helical direction peripherally of a kneading section defining a maximum outside diameter of the screw. Adjacent kneading recesses transversely of the helical direction are connected to one another through flow passages formed.
With the above rotary kneading screw, however, each flow passage must be cut manually adjacent each kneading recess after the plurality of kneading recesses are formed peripherally of the screw. To realize an effective kneading action according to the type and characteristics of material mixture, a cutting technique with a high degree of precision is required which results in a passage cutting operation taking time and trouble. There is also the problem of high manufacturing cost of one rotary kneading screw.